SIMULATION OF NANOCROWN FOR BIOMOLECULAR … · SIMULATION OF NANOCROWN FOR BIOMOLECULAR...

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SIMULATION OF NANOCROWN FOR BIOMOLECULAR PLASMONICS SoonGweon Hong and Luke P. Lee Biomolecular Nanotechnology Center, Berkeley Sensor & Actuator Center Department of Bioengineering, University of California--Berkeley, Berkeley, CA 94720, USA ABSTRACT We present a numerical study of a “nanocrown” plasmonic structure, consisting one main “planet” gold sphere and six satellite gold spheres. The nanocrown has a strong optical enhancement and high spatial density of enhanced hot-spots around the nanocluster, and can increase the intensity of surface-enhanced Raman scattering up to 10 14 by means of the electric field enhancement. Moreover, the nanocluster shows three different modes of local-field enhancement (LFE), and electric field modulation by light incidence angle, which result in high controllability of resonance. This study shows the strong potential of the nanocrown in the field of biomolecular nanoplasmonic sensors and plasmonic optical trapping devices. KEYWORDS: Nanocrown, Localized Surface Plasmon, Local-field enhancement, Surface-enhanced Raman spectroscopy (SERS), Optical trapping INTRODUCTION The remarkable optical properties of nanoscale metal structures have been of great theoretical and experimental interest in recent years due to the locally focused and enhanced electric field around the nanostructures. In particular, the optical properties of metallic structures have been utilized in biological and chemical sensing applications, such as surface-enhanced Raman spectroscopy (SERS) and localized surface plasmon resonance (LSPR) sensing. Recently two main approaches have been investigated to investigate and manipulate these optical properties of nanoscale metal structures. The first relies on the strong field enhancement generated at a sharp metallic tip [1], and the second utilizes coupling between closely spaced metallic nanoparticles [2]. However, the most frequently used techniques, such as electron-beam lithography, show the limitations of spatial resolution and cost-effectiveness in a large area. Therefore, based on a simulation study we suggest the configuration of a seven-nanosphere cluster to increases electric-field enhancement as well as the spatial density of hot-spots (Fig. 1). This cluster can be easily fabricated on a sub-10nm geometric scale on a large area template by mechanical, chemical and electrical self-assembly, which will be described else where. (c) (b) (a) Fig 1. Nanoplasmonic nanocrown. (a) Schematic diagram of nanostructure confined by dielectric material, (b) Parameterization scheme of nanocrown (R 1 : radius of main planet sphere, R 2 : radius of satellite sphere, d 1 : gap distance between main sphere and satellite sphere d 2 : gap distance between each satellite, (c) nanocrown applications of SERS 978-0-9798064-1-4/μTAS2008/$20©2008CBMS 889 Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences October 12 - 16, 2008, San Diego, California, USA

Transcript of SIMULATION OF NANOCROWN FOR BIOMOLECULAR … · SIMULATION OF NANOCROWN FOR BIOMOLECULAR...

  • SIMULATION OF NANOCROWN FOR BIOMOLECULAR PLASMONICS

    SoonGweon Hong and Luke P. Lee Biomolecular Nanotechnology Center, Berkeley Sensor & Actuator Center

    Department of Bioengineering, University of California--Berkeley, Berkeley, CA 94720, USA

    ABSTRACT We present a numerical study of a “nanocrown” plasmonic structure, consisting

    one main “planet” gold sphere and six satellite gold spheres. The nanocrown has a strong optical enhancement and high spatial density of enhanced hot-spots around the nanocluster, and can increase the intensity of surface-enhanced Raman scattering up to 1014 by means of the electric field enhancement. Moreover, the nanocluster shows three different modes of local-field enhancement (LFE), and electric field modulation by light incidence angle, which result in high controllability of resonance. This study shows the strong potential of the nanocrown in the field of biomolecular nanoplasmonic sensors and plasmonic optical trapping devices. KEYWORDS: Nanocrown, Localized Surface Plasmon, Local-field enhancement, Surface-enhanced Raman spectroscopy (SERS), Optical trapping

    INTRODUCTION

    The remarkable optical properties of nanoscale metal structures have been of great theoretical and experimental interest in recent years due to the locally focused and enhanced electric field around the nanostructures. In particular, the optical properties of metallic structures have been utilized in biological and chemical sensing applications, such as surface-enhanced Raman spectroscopy (SERS) and localized surface plasmon resonance (LSPR) sensing. Recently two main approaches have been investigated to investigate and manipulate these optical properties of nanoscale metal structures. The first relies on the strong field enhancement generated at a sharp metallic tip [1], and the second utilizes coupling between closely spaced metallic nanoparticles [2]. However, the most frequently used techniques, such as electron-beam lithography, show the limitations of spatial resolution and cost-effectiveness in a large area. Therefore, based on a simulation study we suggest the configuration of a seven-nanosphere cluster to increases electric-field enhancement as well as the spatial density of hot-spots (Fig. 1). This cluster can be easily fabricated on a sub-10nm geometric scale on a large area template by mechanical, chemical and electrical self-assembly, which will be described else where.

    (c) (b)

    (a) Fig 1. Nanoplasmonic nanocrown. (a) Schematic diagram of nanostructure confined by dielectric material, (b) Parameterization scheme of nanocrown (R1 : radius of main planet sphere, R2 : radius of satellite sphere, d1 : gap distance between main sphere and satellite sphere d2 : gap distance between each satellite, (c) nanocrown applications of SERS

    978-0-9798064-1-4/µTAS2008/$20©2008CBMS 889

    Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

  • THEORY Localized surface plasmon resonance, a collective oscillation of free electrons

    generated by electromagnetic radiation incident on a metallic nanostructure, results in strongly enhanced local optical fields. In the SERS application, the SERS signal intensity is proportional to the 4th power of electric field enhancement

    (4

    0/ EEISERS ∝ ) [3]. To attain both high enhancement at each hot-spot and high number of enhanced spots within a unit volume, a 3-dimensional gold nano-cluster, dubbed the nanocrown, consisting of one main planet sphere and six satellite spheres is suggested here. The asymmetric 3-dimensional configuration with a center sphere causes LSPR focused on the center sphere, resulting in strong hot-spots in the cluster volume. RESULTS AND DISCUSSION

    Analogous to a planet’s gravitational force, the nearby electric field can be attracted to a center nanosphere (which we call a “planet”), resulting in the simultaneous three mode LFEs - (1) large-volume mode (2) strongly-coupled mode and (3) multi-resonance mode. The large-volume mode is lowly enhanced but confined in a large volume by the six satellites and the planet (Fig 2a). The strongly-coupled mode occurs between each satellite and the planet (Fig.2b), which can result in a LFE of up to 350 times and in a SERS enhancement of up to 1012. Considering the number of the clusters placed within a 1μm-diameter laser spot as 102 and simultaneous multi hot-spots in a single cluster, and assuming the chemical enhancement of 103, the nanocrown-SERS substrate increases the SERS signal up to 1018 [4]. The third LFE mode, multi-resonance, is formed between each neighboring satellite (Fig.2c). The spectrum valley between these multi-resonance peaks coincide with the resonance peak of the prior two modes.

    (c) (b) (a) (a)

    (b)

    Fig 2. Three local-field enhancement mode (a) A large-volume mode, (b) A strongly-coupled mode, (c) A multi-resonance mode.

    Fig 3. LFE variation with incident angle. (a) The surface potential and (b) Electric field distribution

    In addition, modulation of the electric-field distribution by the angle of incident light can highlight a specific mode among the three LFE modes. (The angle of incidence is defined relative to the horizontal plane defined by the satellite spheres.) For example, horizontal incidence can intensify the large-volume mode, whereas incidence at an acute angle can accentuate the strongly-coupled mode. In addition, the change of incidence angle shifts the resonance peak of each LFE mode, with the large-volume mode resonance varying from 550nm to 800nm as the incident moves

    890

    Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

  • from 0°to 90°. The geometric parameter variation of the nanocrown presents significant variations in the LFE and in far-field optical properties. Resonance shift can be primarily tuned by changing the size of the planet sphere or satellite spheres (Fig 4). The LFE intensity can be modulated by changing the gap distance between the planet/satellite or between satellites. Far-field optical properties such as extinction are also modulated by the change in these geometric parameters (Fig 5).

    (a) (b)

    (c) (d)

    (a) (b)

    (c) (d)

    Fig 4. Extinction spectra by changing (a) R1, (b) R2,,(c) d1, (d) d2 for normal incidence.

    Fig 5. Extinction spectra of (a) standard shape, (b) R1 change to 60 nm, (c) R2 to 35 nm, (d) d1 to 1 nm. Standard shape is (R1, R2 ,d1, d2)=(40 nm, 20 nm, 5 nm, 5 nm). The color scale of intensity is the same as one of Fig 3.

    CONCLUSIONS

    In summary, we have demonstrated the design and simulation of the nanocrown for biomolecular nanoplasmonic sensors and advanced plasmonic optical trapping devices. Especially, combination of strongly-coupled mode and large-volume mode resulting in comparatively big size of hot spot, is expected to provide highly enhanced SERS signals with reduced signal fluctuation during measurement. Additionally, the nanocrown generates unique nano-optical properties, such as the simultaneous three different modes of local-field enhancement, electric-field modulation by light incidence angle and resonance peak shifting by changes of geometric parameters.

    ACKNOWLEDGEMENTS

    This work was supported by a grant (code #: 05K1501-02810) from ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Programs’ of the Ministry of Science and Technology, Korea, and from DARPA. REFERENCES [1] L. Novotny, et al., Phys. Rev. Lett. 79, 645 (1997) [2] J. Kottmann and O. J. F Martin, Opt. Express 8, 655 (2001) [3] C. L. Haynes, et al., J. Phys. Chem. B 107, 7337 (2003) [4] C. E. Talley, et al., Nano Letters 5, 1569 (2005)

    891

    Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

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